Compression Test - 8.3:1 Wiseco?

[mnetwork]

I have a built motor with 7bolt block, Mitsu MLS head gasket, Wiseco 8.3:1 (.020 overbore) pistons, stock valves, and BC272 cams. The motor has a few thousand miles on it now andf I just did a compression test. All cylinders were tested twice and all tests were done with motor > 183 degrees and with throttle WOT.

My numbers were:

Cyl1 - 165
Cyl2 - 172 (little lower than the 175 mark)
Cyl3 - 165
Cyl4 - 165

Was wondering what I should make of these.

 

[LiquidX]

Although #2 is higher, they are all great numbers, nice and even across the board.

 

[99gst_racer]

Agreed. They look good to me too.

 

[mnetwork]

These numbers aren't too low?? Motor was built end of last august, but has a good amount less than 10k.

What do you think would cause Cyl 2 to be slightly higher? I even went back to Cyl 2 at the end just to make sure it wasn't a fluke.

 

[99gst_racer]

Carbon build-up could cause a slight compression bump in a single cylinder. We're only talking a difference of 7 psi, so it's definitely not a deficit to be concerned with. Those compression numbers sound accurate for 8.3:1 pistons and 272 cams.

 

[LiquidX]

My last build had a healthy 150psi across the board after the break in.

 

[ed1380]

What is the difference between good and bad compression? From what I've been taught 130 and below by the book requires a rebuild. and 180 is a good number to aim for

 

[LiquidX]

That's also on stock internals.

 

[jlr2196]

Longer duration cams such as 272's will bleed off compression, this is why you should run higher compression when running larger cams. You could build a 10:1 engine with huge 284 duration cams and show less compression on a compression test than a 8:1 engine with stock cams.

 

[mnetwork]

Carbon build-up could cause a slight compression bump in a single cylinder. We're only talking a difference of 7 psi, so it's definitely not a deficit to be concerned with. Those compression numbers sound accurate for 8.3:1 pistons and 272 cams.

My last build had a healthy 150psi across the board after the break in.

Thanks guys, glad to hear it looks good. I'm doubting carbon since the motor runs so clean with E85, but who knows. Maybe some left over from the few thousand miles I ran 93...

Longer duration cams such as 272's will bleed off compression, this is why you should run higher compression when running larger cams. You could build a 10:1 engine with huge 284 duration cams and show less compression on a compression test than a 8:1 engine with stock cams.

I've heard cams bleed off, but I never verified that that was really so...